Free-space optical communications, light detection and ranging (lidar), and optical scanning all require some means of steering optical beams. Usually, fast steering and large scan angles are the two most important considerations for these applications. In some cases, the beams are hopped rather than steered from location to location, a feature sometimes known as agile beam steering.
Perhaps the most common way to steer optical beams is by reflecting the beams off mirrors or diffracting off holographic gratings mounted on mechanical scanners, such as rotating prisms, nodding scanners, and spinning or translating mounts. Mechanical scanners can easily scan beams over large angles at kilohertz rates, but steering a single beam in two dimensions requires two moving mirrors or gratings, each rotating or moving about a different axis. Steering multiple beams in two dimensions may require one pair of moving mirrors for each beam, depending on the application. Galvanometers and piezoelectric scanners tend to be bulky and generally require high-current and high-voltage servo drivers, respectively. Position drift dues to temperature fluctuations limits the pointing accuracy, as does hysteresis in the response to the servo driver.
Pairs of independently rotating prisms, such as Risley prisms, use refraction to steer optical beams over a potentially continuous range of angles. Although Risley prisms can be mounted compactly along independent rotational axes, the prisms and mounts have extremely tight manufacturing tolerances. For example, imprecisely matched prisms may not be able to steer beams along the optical axis. Further, even perfectly matched prisms must be rotated through nearly 180° to steer the beam over angles close to the optical axis. In addition, chromatic aberrations inherent in dispersive optics cause wavelength-dependent deviations in the steering angle.
Acousto-optic deflectors (AODs) can also scan beams over relatively large angles at megahertz rates. Like mechanical scanners, AODs tend to be bulky and can only steer beams in one dimension. AODs, however, can easily operate at Megahertz rates, although they need radio-frequency drive signals with power levels on the order of 500 mW. Because AODs diffract rather than reflect the incident beam, the deflection angle depends in part on the wavelength of the incident beam. AODs also diffract multiple orders (even when Bragg-matched), introducing unwanted beams into the scene being illuminated.
Micro-electro-mechanical systems (MEMS) are also used for steering and deflecting optical beams. Although MEMS devices are quite compact, their angular field-of-view in any one dimension tends to be no more than a few degrees at most. MEMS device cannot operate at rates much faster than a few kilohertz. In addition, surface effects, such as stiction (i.e., static friction), can cause MEMS components to stick in certain positions during fabrication and operation, effectively ruining the devices.
Beam steering with liquid-crystal spatial light modulators (SLMs) also tends to suffer from limited field-of-view and slow (kilohertz) steering speeds. In SLM-based scanners, a laser beam illuminates an SLM, which transmits (or reflects, depending on the device geometry) a version of the incident beam imprinted with a spatial phase modulation according to a drive signal applied to the SLM. The resulting spatial phase modulation of the transmitted (or reflected) laser beam can be transformed into a transverse displacement with a lens. Because the SLM uses polarization effects to imprint spatial phase modulation on the incident beam, only a fraction of the incident beam is transmitted (or reflected), dramatically reducing the optical transmission efficiency of the system.
Another approach to beam steering involves transmitting light through a series of cascaded decentered lenses. A first lens focuses a collimated beam to a spot in its back focal plane, which coincides with the front focal plane of a second lens. If the second lens is offset with respect to the first lens, the second lens will project a collimated beam at an angle determined by the ratio of the offset to the lens focal length. Moving the second lens sideways causes the projection angle to change, scanning the beam. Even though the scanner can be made smaller by replacing the lenses with microlens arrays, the scanner still uses moving parts. As a result, its scanning speed will be limited to kilohertz rates, at best.
In the radio-frequency domain, phased array antennas control beam angles by adjusting the relative phases of beams emitted at each antenna element. At optical frequencies, however, coherent control is quite difficult because the wavelength is so short (usually about 1 μm). Cohering multiple independent sources, even using injection-locking techniques, usually does not provide long-term phase stability needed for most beam steering applications. To see why not, consider a set of lasers emitting at a wavelength of λ=850 nm. To be coherently combined, the lasers must be phase-locked to within a fraction (no more than 5-10%) of a single period, which is about 2 ps for λ=850 nm. Similarly, splitting a beam from a single laser requires path-length matching to within a fraction of a wavelength (typically about 5-10%, or 40 nm for λ=850 nm). Unfortunately, environmental perturbations, such as thermal and mechanical drift, make it impractical to match paths to within these lengths.